(1) Field of the Invention
The present invention relates to an optical disc apparatus, an information recording method, and an optical disc that are employed to record information using laser light and particularly to a recording technique suitable for high-speed recording.
(2) Description of the Related Art
Optical discs are information recording media on/from which information can be written/read. Write-once optical discs include CD-Rs, DVD-Rs, DVD+Rs, BD-Rs, and so forth. Optical discs that allow multiple recordings are, for example, CD-RWs, DVD-RAMs, DVD-RWs, DVD+RWs, and BD-REs. Especially high-capacity recording media are BDs (Blu-ray Discs), which involve the use of blue laser light for data recording/reading. For instance, 12-cm BDs can store 25-GB data per recording layer. Also, for the purpose of high-density recording, BDs adopt the data modulation method in which data lengths vary from 2 T to 9 T relative to the reference clock length 1 T.
To record information on an optical disc, the information is converted into a data stream having 2 T to 9 T data sets, and recording marks are formed based on the data stream. Often, one mark is formed using not one laser pulse but multiple short pulses. This method is called a write strategy and serves to prevent heat accumulation on the recording surface of the optical disc, thus allowing formation of accurate marks.
There are two types of write strategies: 1) N−1 write strategy and 2) castle write strategy.
1) In the N−1 write strategy, the number of write pulses is increased as the length of write data increases. For example, 2 pulses are used for forming a 3 T mark, and 3 pulses for a 4 T mark, and so on. Thus, the write pulses used for forming a 5 T or longer mark comprise a top pulse, multiple middle pulses, and a last pulse. In the case of BD-Rs and other write-once optical discs, the length of a top pulse is often made longer than those of other pulses in order to apply sufficient thermal energy, and the lengths of the subsequent multiple pulses and last pulse are set short enough to retain the applied thermal energy.
2) The castle write strategy uses a top pulse, a bias pulse (also called a middle pulse), and a last pulse. The top pulse has a given write power level, and the bias pulse has a write power level lower than that of the top pulse. The last pulse has a write power level which is equal to that of the top pulse. The middle bias pulse is used to retain thermal energy. Because the castle write strategy allows write pulse rise and fall times to be shortened, recording waveforms are less subject to deterioration. Thus, the castle write strategy is suitable for high-speed recording on BD-Rs and the like.
Such write strategies as above are disclosed in Japanese Laid-Open Unexamined Application No. 2003-85753, for example.